Automatic refrigerator temperature controller. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers
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The machine allows you to maintain the temperature in the refrigerator at a given level.
The sensitive element of the machine is the thermistor R1, included in one of the arms of the measuring bridge. A constant voltage is supplied to one diagonal of the bridge from a half-wave rectifier on the diode V7. The unbalance voltage, taken from the other diagonal of the bridge, is fed to a current amplifier made on transistors V1 and V2. When the bridge is balanced, there is no voltage at its output, relay K.1 is in a de-energized state and the refrigerator is connected to the network through normally closed contacts K1.1. Cooling to a given temperature is set using resistor R2. A decrease in temperature compared to the set one leads to an unbalance of the bridge. The unbalance voltage, amplified by transistors V1 and V2, turns on relay K1, and the refrigerator will be disconnected from the network through the contacts of relay K1.1 and connector X2.
Details. Transistors type MP39-MP42 with h21e > 20; relay type RKN or others, designed for an actuation voltage of 10-15 V at a current of 5-10 mA and a switching current of up to 0,5 A at 220 V; the transformer is made on a magnetic circuit Ш20 x 25: winding I contains 2200 turns of PEV-0,15 wire, winding II - 220 turns of PEV-1 0,25, winding III - 350 turns of PEV-1 0,1.
The scale of the resistor R2 is calibrated by a reference thermometer located next to the thermistor in the refrigerator chamber. By setting the resistor knob to two extreme positions, approximately 1 hour after turning on the refrigerator, determine the temperature of the refrigerator compartment. Similarly, calibrate the intermediate points of the resistor scale. If a thermistor of a different type is used, the resistances of all resistors of the measuring bridge must match the resistance of the thermistor and satisfy the condition R1R2=(R1+ R2)R4.
Author: B.Ivanov
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